Zinc oxide (flowers of zinc) has long been known as a white pigment. Zinc oxide exhibits the following optical properties. When zinc oxide is formed into fine particles having a diameter approximately half the wavelength of visible light, the particles allow visible Light to pass therethrough since the scattering effect of the zinc dioxide particles is reduced considerably, and selectively absorb ultraviolet light by virtue of the excellent ultraviolet absorbing effect of zinc oxide.
Conventionally known ultraviolet absorbers include organic ultraviolet absorbers such as benzophenone-based absorbers, benzotriazole-based absorbers, salicylate-based absorbers, and substituted-acrylonitrile-based absorbers. However, a limitation is imposed on use of such an ultraviolet absorber, from the viewpoint of safety. Meanwhile, when such an ultraviolet absorber is incorporated into a thermoplastic resin or a similar material and the resultant mixture is subjected to molding, the absorber is decomposed because of its poor heat resistance or undergoes bleedout during molding. Therefore, zinc oxide particles exhibiting excellent safety and high heat resistance have become of interest as a replacement for organic ultraviolet absorbers.
Regarding ultraviolet absorbers containing such zinc oxide particles, Japanese Patent Application Laid-Open (kokai) No. 5-171130 discloses a resin molded product in which zinc oxide fine powder having a particle size of 0.1 μm or less is incorporated into a transparent resin. Japanese Patent Application Laid-Open (kokai) Nos. 5-295141 and 11-302015 disclose zinc oxide fine particles which are coated with a silicon compound, in order to reduce impairment of weather resistance of the fine particles attributable to the photocatalytic effect, and to improve dispersibility of the fine particles in a resin.
In addition, Japanese Patent No. 2501663 (International patent publication W090/06974) proposes a method for encapsulation of zinc oxide pigment composition by coating the composition with a deposit of a water insoluble metallic soap. In this method, a water-soluble alkali metal salt of saturated or unsaturated monocarboxylic acid having 7 to 22 carbon atoms and a water-soluble metallic salt consisting of a metallic cation and an inorganic anion portions are added to a slurry comprising a pigment zinc oxide composition, wherein the metallic cation is selected from IB, II, III, IV, V, VIB, VIIB or VIII group of the periodic table, and the inorganic anion portion is selected from a group consisting of a nitrate ion, a sulphate ion, and a halogen ion; and thus the water-soluble metallic soap of the saturated or unsaturated monocaboxylic acid is formed and deposited there.
However, conventional zinc oxide particles produced through the techniques disclosed in the above publications aggregate considerably and are difficult to disperse uniformly in a resin. Therefore, the resultant resin composition becomes turbid, and does not necessarily exhibit sufficient transparency.
Furthermore, a method using a solvent such as water or an organic solvent in the surface treatment requires a step of filtration and drying of the solvent, and tends to cause an uneven distribution of the surface treatment agent deposited by drying and aggregation of powders. Therefore, this method has a disadvantage in that excellent dispersiblity of the coated zinc oxide particles is difficult to obtain.